PROJECT SUMMARY Parent-mediated interventions often target social communication in young children with ASD, although to date studies yield inconsistent effects. One reason for the limited evidence may be the considerable heterogeneity in both parent and child characteristics that affect the fit of intervention to family and ultimately influence treatment outcome. For parents, these factors might include stress associated with the uncertainty of their child's diagnosis, caregiver expectations for the intervention itself, and a parent's own style of interaction that may be influenced by milder but qualitatively similar ASD characteristics, known as the broad autism phenotype (BAP). For children, these factors might include nonverbal DQ, language, or sensory impairment. The fit between type of intervention and optimal outcome for parent and child is an understudied, yet essential component of early intervention that may be susceptible to the influence of heterogeneity in the parent and child. One approach to addressing this variability is to implement an adaptive intervention approach that seeks to capitalize on heterogeneity among children and parents. Utilizing an adaptive treatment design, the current study tests the optimal sequence of intervention delivery and specific parent and child characteristics that may moderate treatment success in three 10-week stages of intervention. The first phase will randomize parents and children to a parent education condition, consisting of a parent support and education group focused on social communication development, or to a parent mediated and therapist delivered condition involving coaching of the parent with their child in social communication strategies. Phase 2 involves re-randomizing parents and children to maintain the same treatment arm, or change to the opposite arm to test the optimal sequence of intervention delivery and specific parent and child characteristics that may moderate treatment success. In the final phase, dyads are randomized to different maintenance arms, each comprised of 5 sessions with one involving skype and text contact, the other in -home visits, to explore how best to maintain treatment gains once the active intervention phase is complete. This study has the potential to dramatically improve child social communication outcomes by individualizing and personalizing parent intervention approaches with very young children, a high priority need of the Interagency Autism Coordinating Council and NIH.